This invention relates to the positive displacement pumping of liquids, and more particularly to methods and apparatus for metering relatively small quantities of liquid, such as when a chemical solution is fed from a supply container into a large body or stream of liquid which may or may not be pressurized.
Small metering pumps have used spring biased check valves to control liquid intake and recirculation flow. The springs had to be located in liquid flow conduits, which caused them to corrode, or contaminate the liquid, or otherwise malfunction. Such springs and the outlet check valve spring had to be relatively weak (e.g., 2 oz.) in order to permit priming, suction lift and degassing of the pump to occur against the lightly biased check valves. When the pump was used to meter solutions containing foreign matter or chemicals which build up on or around the check valves, the valves would soon cease to operate. Also, the quantity of liquid dispensed by such pumps could not always be accurately controlled as the springs or valve sealing surfaces deteriorated. Sometimes a chemical solution being pumped would form a precipitate that would cause a blockage downstream from the pump, and continued operation of the pump would cause internal pressure to build up until the pump or its motor were damaged.